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• The extent to which water availability can be used to predict the enlargement and final dimensions of xylem conduits remains an open issue. • We reconstructed the time course of tracheid enlargement in Pinus sylvestris trees in central Spain by repeated measurements of tracheid diameter on microcores sampled weekly during a 2 yr period. We analyzed the role of water availability in these dynamics empirically through time-series correlation analysis and mechanistically by building a model that simulates daily tracheid enlargement rate and duration based on Lockhart’s equation and water potential as the sole input. • Tracheid enlargement followed a sigmoid-like time course, which varied intra- and interannually. Our empirical analysis showed that final tracheid diameter was strongly related to water availability during tracheid enlargement. The mechanistic model was calibrated and successfully validated (R² = 0.92) against the observed tracheid enlargement time course. The model was also able to reproduce the seasonal variations of tracheid enlargement rate, duration and final diameter (R² = 0.84–0.99). • Our results support the hypothesis that tracheid enlargement and final dimensions can be modeled based on the direct effect of water potential on turgor-driven cell expansion. We argue that such a mechanism is consistent with other reported patterns of tracheid dimension variation.

• Trait variability in space and time allows plants to adjust to changing environmental conditions. However, we know little about how this variability is distributed and coordinated at different organizational levels. • For six dominant tree species in northeastern Spain (three Fagaceae and three Pinaceae) we quantified the inter- and intraspecific variability of a set of traits along a water availability gradient. We measured leaf mass per area (LMA), leaf nitrogen (N) concentration, carbon isotope composition in leaves (δ13C), stem wood density, the Huber value (Hv, the ratio of cross-sectional sapwood area to leaf area), sapwood-specific and leaf-specific stem hydraulic conductivity, vulnerability to xylem embolism (P 50) and the turgor loss point (P tlp). • Differences between families explained the largest amount of variability for most traits, although intraspecific variability was also relevant. Species occupying wetter sites showed higher N, P 50 and P tlp, and lower LMA, δ13C and Hv. However, when trait relationships with water availability were assessed within species they held only for Hv and P tlp. • Overall, our results indicate that intraspecific adjustments along the water availability gradient relied primarily on changes in resource allocation between sapwood and leaf area and in leaf water relations.

• Clarifying the coordination of leaf hydraulic traits with gas exchange across closely-related species adapted to varying rainfall can provide insights into plant habitat distribution and drought adaptation. • The leaf hydraulic conductance (K leaf), stomatal conductance (g s), net assimilation (A), vein embolism and abscisic acid (ABA) concentration during dehydration were quantified, as well as pressure–volume curve traits and vein anatomy in 10 Caragana species adapted to a range of mean annual precipitation (MAP) conditions and growing in a common garden. • We found a positive correlation between Ψleaf at 50% loss of K leaf (K leaf P 50) and maximum K leaf (K leaf-max) across species. Species from low-MAP environments exhibited more negative K leaf P 50 and turgor loss point, and higher K leaf-max and leaf-specific capacity at full turgor, along with higher vein density and midrib xylem per leaf area, and a higher ratio of K leaf-max : maximum g s. Tighter stomatal control mediated by higher ABA accumulation during dehydration in these species resulted in an increase in hydraulic safety and intrinsic water use efficiency (WUEi) during drought. • Our results suggest that high hydraulic safety and efficiency combined with greater stomatal sensitivity triggered by ABA production and leading to greater WUEi provides drought tolerance in Caragana species adapted to low-MAP environments.

• Although xylem embolism resistance is traditionally considered as static, we hypothesized that in grapevine (Vitis vinifera) leaf xylem becomes more embolism-resistant over the growing season. • We evaluated xylem architecture, turgor loss point (ΨTLP) and water potentials leading to 25% of maximal stomatal conductance (g s25) or 50% embolism in the leaf xylem (P50) in three irrigation treatments and at three time points during the growing season, while separating the effects of leaf age and time of season. • Hydraulic traits acclimated over the growing season in a coordinated manner. Without irrigation, ΨTLP, g s25, and P50 decreased between late May and late August by 0.95, 0.77 and 0.71 MPa, respectively. A seasonal shift in P50 occurred even in mature leaves, while irrigation had only a mild effect (< 0.2 MPa) on P50. Vessel size and pit membrane thickness were also seasonally dynamic, providing a plausible explanation for the shift in P50. • Our findings provide clear evidence that grapevines can modify their hydraulic traits along a growing season to allow lower xylem water potential, without compromising gas exchange, leaf turgor or xylem integrity. Seasonal changes should be considered when modeling ecosystem vulnerability to drought or comparing datasets acquired at different phenological stages.

Under climate change, extreme droughts will limit water availability for plants. However, the species-specific responses make it difficult to draw general conclusions. We hypothesized that changes in species’ abundance in response to extreme drought can be best explained by a set of water economic traits under ambient conditions in combination with the ability to adjust these traits towards higher drought resistance. We conducted a 4-year field experiment in temperate grasslands using rainout shelters with 30% and 50% rainfall reduction. We quantified the response as the change in species abundance between ambient conditions and the rainfall reduction. Abundance response to extreme drought was best explained by a combination of traits in ambient conditions and their functional adjustment, most likely reflecting plasticity. Smaller leaved species decreased less in abundance under drought. With increasing drought intensity, we observed a shift from drought tolerance, i.e., an increase in leaf dry matter content, to avoidance, i.e., a less negative turgor loss point (TLP) in ambient conditions and a constancy in TLP under drought. We stress the importance of using a multidimensional approach of variation in multiple traits and the importance of considering a range of drought intensities to improve predictions of species’ response to climate change.

Given increasing water deficits across numerous ecosystems world-wide, it is urgent to understand the sequence of failure of leaf function during dehydration. We assessed dehydration-induced losses of rehydration capacity and maximum quantum yield of the photosystem II (F v/F m) in the leaves of 10 diverse angiosperm species, and tested when these occurred relative to turgor loss, declines of stomatal conductance g s, and hydraulic conductance K leaf, including xylem and outside xylem pathways for the same study plants. We resolved the sequences of relative water content and leaf water potential Ψleaf thresholds of functional impairment. On average, losses of leaf rehydration capacity occurred at dehydration beyond 50% declines of g s, K leaf and turgor loss point. Losses of F v/F m occurred after much stronger dehydration and were not recovered with leaf rehydration. Across species, tissue dehydration thresholds were intercorrelated, suggesting trait co-selection. Thresholds for each type of functional decline were much less variable across species in terms of relative water content than Ψleaf. The stomatal and leaf hydraulic systems show early functional declines before cell integrity is lost. Substantial damage to the photochemical apparatus occurs at extreme dehydration, after complete stomatal closure, and seems to be irreversible.

Predicting responses of tropical forests to climate change-type drought is challenging because of high species diversity. Detailed characterization of tropical tree hydraulic physiology is necessary to evaluate community drought vulnerability and improve model parameterization. Here, we measured xylem hydraulic conductivity (hydraulic efficiency), xylem vulnerability curves (hydraulic safety), sapwood pressure–volume curves (drought avoidance) and wood density on emergent branches of 14 common species of Eastern Amazonian canopy trees in Paracou, French Guiana across species with the densest and lightest wood in the plot. Our objectives were to evaluate relationships among hydraulic traits to identify strategies and test the ability of easy-to-measure traits as proxies for hard-to-measure hydraulic traits. Xylem efficiency was related to capacitance, sapwood water content and turgor loss point, and other drought avoidance traits, but not to xylem safety (P 50). Wood density was correlated (r = −0.57 to −0.97) with sapwood pressure–volume traits, forming an axis of hydraulic strategy variation. In contrast to drier sites where hydraulic safety plays a greater role, tropical trees in this humid tropical site varied along an axis with low wood density, high xylem efficiency and high capacitance at one end of the spectrum, and high wood density and low turgor loss point at the other.

Identifying key traits that can serve as proxies for species drought resistance is crucial for predicting and mitigating the effects of climate change in diverse plant communities. Turgor loss point (πtlp) is a recently emerged trait that has been linked to species distributions across gradients of water availability. However, a direct relationship between πtlp and species ability to survive drought has yet to be established for woody species. Using a manipulative field experiment to quantify species drought resistance (i.e., their survival response to drought), combined with measurements of πtlp for 16 tree species, we show a negative relationship between πtlp and seedling drought resistance. Using longterm forest plot data, we also show that πtlp predicts seedling survival responses to a severe El Niño-related drought, although additional factors are clearly also important. Our study demonstrates that species with lower πtlp exhibit higher survival under both experimental and natural drought. These results provide a missing cornerstone in the assessment of the traits underlying drought resistance in woody species and strengthen πtlp as a proxy for evaluating which species will lose or win under projections of exacerbating drought regimes.

Intracellular NHX proteins are Na⁺,K⁺/H⁺ antiporters involved in K⁺ homeostasis, endosomal pH regulation, and salt tolerance. Proteins NHX1 and NHX2 are the two major tonoplast-localized NHX isoforms. Here, we show that NHX1 and NHX2 have similar expression patterns and identical biochemical activity, and together they account for a significant amount of the Na⁺,K⁺/H⁺ antiport activity in tonoplast vesicles. Reverse genetics showed functional redundancy of NHX1 and NHX2 genes. Growth of the double mutant nhx1 nhx2 was severely impaired, and plants were extremely sensitive to external K⁺. By contrast, nhx1 nhx2 mutants showed similar sensitivity to salinity stress and even greater rates of Na⁺ sequestration than the wild type. Double mutants had reduced ability to create the vacuolar K⁺ pool, which in turn provoked greater K⁺ retention in the cytosol, impaired osmoregulation, and compromised turgor generation for cell expansion. Genes NHX1 and NHX2 were highly expressed in guard cells, and stomatal function was defective in mutant plants, further compromising their ability to regulate water relations. Together, these results show that tonoplast-localized NHX proteins are essential for active K⁺ uptake at the tonoplast, for turgor regulation, and for stomatal function.

Quantifying drought tolerance in crops is critical for agriculture management under environmental change, and drought response traits in grape vine have long been the focus of viticultural research. Turgor loss point (Ï.sub.tlp) is gaining attention as an indicator of drought tolerance in plants, though estimating Ï.sub.tlp often requires the construction and analysis of pressure-volume (P-V) curves which are very time consuming. While P-V curves remain a valuable tool for assessing Ï.sub.tlp and related traits, there is considerable interest in developing high-throughput methods for rapidly estimating Ï.sub.tlp, especially in the context of crop screening. We tested the ability of a dewpoint hygrometer to quantify variation in Ï.sub.tlp across and within 12 clones of grape vine (Vitis vinifera subsp. vinifera) and one wild relative (Vitis riparia), and compared these results to those derived from P-V curves. At the leaf-level, methodology explained only 4-5% of the variation in Ï.sub.tlp while clone/species identity accounted for 39% of the variation, indicating that both methods are sensitive to detecting intraspecific Ï.sub.tlp variation in grape vine. Also at the leaf level, Ï.sub.tlp measured using a dewpoint hygrometer approximated Ï.sub.tlp values (r.sup.2 = 0.254) and conserved Ï.sub.tlp rankings from P-V curves (Spearman's [rho] = 0.459). While the leaf-level datasets differed statistically from one another (paired t-test p = 0.01), average difference in Ï.sub.tlp for a given pair of leaves was small (0.1 ± 0.2 MPa (s.d.)). At the species/clone level, estimates of Ï.sub.tlp measured by the two methods were also statistically correlated (r.sup.2 = 0.304), did not deviate statistically from a 1:1 relationship, and conserved Ï.sub.tlp rankings across clones (Spearman's [rho] = 0.692). The dewpoint hygrometer (taking \\ 10-15 min on average per measurement) captures fine-scale intraspecific variation in Ï.sub.tlp, with results that approximate those from P-V curves (taking 2-3 h on average per measurement). The dewpoint hygrometer represents a viable method for rapidly estimating intraspecific variation in Ï.sub.tlp, and potentially greatly increasing replication when estimating this drought tolerance trait in grape vine and other crops.